A very wide variety of telephone sets used to initiate and receive telephone calls has been shown in the field of telecommunications. While improvements in the field have introduced significant advances in the design and capabilities of telephone sets, even the simplest telephone, whether electronic or electro-mechanical in nature, must perform several fundamental tasks. Among these basic operations are: requesting use of the telephone system when the handset is lifted or the hook-switch is closed; indicating that the system is ready for use by receiving and reproducing a dial tone; sending the number of the telephone to be called to the system; indicating the status of a call in progress (e.g. "ringing", "busy", etc. . . .) by receiving and reproducing tones indicative of the status; indicating an incoming call by means of a ringer or other audible or visual signal generator; converting the speech of a calling party into electrical signals for transmission to a distant party through the system, and converting electrical signals received from a distant party into acoustical signals; adjusting automatically to variations in the power supplied to it; and signalling the system that a call is finished when a caller "hangs up".
The connection between a calling party and a called party is established by the action of a central switching facility. Each telephone is connected to a central switching station containing signalling equipment and batteries that supply direct current (DC) power to operate the telephone set. In typical systems, each phone is connected to the central switching facility through a local loop of two wires called a wire pair or a "twisted pair". Switches in the switching station respond to dial pulses or dial tones from the telephone to connect the calling phone to the called phone. When the connection is established, the two telephones communicate by means of a current loop using current supplied by the central station's batteries.
With a conventional electro-mechanical telephone, a handset containing a receiver and a microphone rests in a cradle when the phone is not in use. The weight of the handset holds the hook-switch down, leaving the hook-switch open. In this position, called an "on-hook" condition, no current loop is established on the wire pair between the telephone set and the central switching station; however, a ringer circuit in the telephone is always connected to the wire pair from the central office, so that the telephone may detect incoming calls heralded by an AC ringing signal on the phone lines.
When the handset of an electro-mechanical telephone set is removed from its cradle, the spring-loaded hook-switch closes, completing the circuit with the central station. This is called an "off-hook" condition. The central switching station detects the newly established flow of current in the loop and issues a dial tone on the lines to indicate that the calling party can enter a telephone number specifying the called party.
Telephone numbers can be communicated to the central station either in the form of dialing pulses or dialing tones. Dialing pulses are generated by repeatedly opening and closing the hook-switch; dialing tones are generated by an electronic tone generator. The central switching station includes detection circuitry for recognizing dialing pulses or dialing tones, allowing it to automatically establish the connection to the phone specified by the caller. If the called phone handset is "off-hook" when the connection is requested, a busy tone generated by the central station is returned to the calling phone. Otherwise, a ringing signal is sent to the called phone, and a "ringback" tone is returned to the calling phone to indicate that the called phone is ringing.
When the called party removes the handset in response to a ringing signal, the loop to that phone is completed by its closed hook-switch and loop current flows between that phone and the central station. The central office then removes the ringing signal and the ringback tone from the circuit.
The transmitter of a telephone converts the acoustical signals of a caller's speech into variations in the electrical current flowing in the current loop. The receiver of a telephone converts variations in electrical current flowing in the current loop back into acoustical signals audible to a user. A small amount of transmitted signal, called sidetone signal, is fed back into the receiver of the transmitting telephone set; this allows the user to hear his or her own voice in the receiver to determine how loudly to speak.
A telephone call is terminated when either the called or calling party "hangs up" the handset, putting the called or calling telephone in an "on-hook" condition. The central switching station detects the interruption of loop current, and releases the connection.
The aforementioned basic functions are performed by even the simplest telephone sets. It has proven to be desireable to provide telephones having certain advanced features that make them suitable for particular applications or that make the telephone generally easier and more efficient to use. For example, one popular feature to include in telephones is the ability to store frequently dialed telephone numbers in a memory, and retrieve and issue these numbers on command. Such a feature is called memory dialing, and various memory-dialing telephones have been shown in the art. Similar feature is the so-called "emergency dialing" feature, in which emergency numbers can be dialed with a limited number (e.g. one or two) keystrokes. Another popular feature incorporated into telephone sets is a transmit mute function, which enables a user to momentarily prevent sound from being transmitted from the telephone. This feature is similar to the conventional "hold" function often found on commercial-use telephone sets, except that received signals from the called party are still audible from the muted handset.
Often, full-featured telephone sets are partially or completely electronic in design, since features such as memory dialing are best implemented using semiconductor logic devices In electronic telephone sets, operational power for the various electronic components may either be provided from a standard household electrical receptacle, or directly from the batteries provided at a central switching station In the former case, an AC-to-DC transformer is required for converting household AC power into DC power suitable for operating electronic devices. In the latter case, called a line powered unit, no AC-to-DC transformer is needed, since the DC power is available on the incoming wire pair; line powered electronic telephone sets may therefore be less expensive, smaller, and lighter than electronic telephones having an AC-to-DC transformer. In designing electronic telephone sets, however, particular attention must be paid to the power consumption characteristics, in order that they not consume too much power from the central station Low power consumption is particularly desireable during "on-hook" operation.
A further consideration in the design of line-powered electronic telephone sets is that they should be very tolerant of variations in power supplied from the central switching station's batteries, and they should be resistant to voltage surges or spikes which are liable to occur on the wire pair
It is an object of the present invention to provide a telephone which is equipped with several of the aforementioned features. In addition, however, it is an object of the present invention to incorporate such advanced features in such a way that results in a telephone that is particularly well-adapted for use by persons with visual and hearing impairments, or by persons wearing hearing aids.
It is still another object of the present invention to provide a full-featured telephone which is line-powered, but which is resistant to noise and voltage surges which may appear on the local loop. A further object of the present invention is to provide a telephone which is capable of storing a plurality of memory-dialing numbers in its memory, and which retains this memory even when the telephone is temporarily disconnected from the telephone lines.